Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session MU: Separated Flows II and Miscellaneous Experiments |
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Chair: Michael Olsen, Iowa State University Room: 200I |
Tuesday, November 24, 2009 8:00AM - 8:13AM |
MU.00001: Control of Flow past a Stationary Circular Cylinder: Effects of a Single Straight Wire and a System of Helical Wires Alis Ekmekci, Donald Rockwell Effects of geometric disturbances on the surface of a circular cylinder are investigated experimentally via Particle Image Velocimetry (PIV). Consideration is given to a single straight wire and a system of three equidistant helical wires. The Reynolds number is 10,000 and the scale of the wire is about one percent of the cylinder diameter; moreover, the wire diameter is smaller than the thickness of the unperturbed boundary layer developing around the cylinder. A straight wire, located at an angle of 60$^\circ $ from the forward stagnation point of the cylinder, yields bistable shear layer oscillations between two distinct states involving reattachment and no reattachment downstream of separation at the wire. For the helical wire system, bistable oscillations also occur at the flow cross-section where the helical wire passes through this same critical angular location on the cylinder surface. These types of small-scale geometric disturbances do not attenuate the Karman instability in the presence of bistable oscillations. However, they markedly influence the onset of instability of the separated shear layer. [Preview Abstract] |
Tuesday, November 24, 2009 8:13AM - 8:26AM |
MU.00002: Manipulating the Forces on a Sphere Using a Dynamic Roughness Element A.K. Norman, B.J. McKeon Though the effect of distributed roughness on flow over a sphere has been examined in detail, there have been few observations as to the effect of an isolated roughness element on the forces induced on a sphere that is in uniform flow. In this experimental study, we examine how the forces are altered due to both a stationary and dynamic three-dimensional roughness element in the Reynolds number range of $5\times10^4$ to $5\times10^5$. It is found that even a small change to the geometry of the sphere, by adding a cylindrical roughness element with a width and height of $1\%$ the sphere diameter, dramatically alters the drag and lateral forces over a wide range of Reynolds numbers. Of particular interest is that the mean of the lateral force magnitude can be increased by a factor of about seven, compared with a stationary roughness element, by moving the isolated roughness at a constant angular velocity about the sphere. The interaction of the roughness element with the flow is examined to understand the cause of the large forces. [Preview Abstract] |
Tuesday, November 24, 2009 8:26AM - 8:39AM |
MU.00003: Control of the Near-Wake of a Forced-Oscillating Cylinder via a Straight Surface Wire Tayfun Aydin, Veitch Thomas, Alis Ekmekci Effects of a spanwise surface wire on the near-wake of a circular cylinder subjected to small-amplitude, forced- oscillation at the inherent Karman frequency are studied via Particle Image Velocimetry (PIV). The Reynolds number is 10,000 and the diameter of the surface wire is two orders of magnitude smaller than the cylinder diameter, but larger than the unperturbed boundary layer thickness prior to its separation. The near wake is markedly affected by the wire when it is located within a certain range of angular positions with respect to the approach flow. As the angular position is altered, the near wake undergoes a contraction, followed by an extension in the streamwise direction. This trend is in the reverse order of what has been reported for a stationary cylinder. The predominant frequency of the velocity fluctuations in the separating shear layers locks onto the frequency of the forced oscillation and remains unchanged with the angular position of the wire. However, the predominant frequency in the near wake shows a gradual decrease, then increase, as the angular position of the wire is increased within the defined range of angles. [Preview Abstract] |
Tuesday, November 24, 2009 8:39AM - 8:52AM |
MU.00004: An Experimental Study of Flow Around a Spinning Cylinder Cesar Cantu, Benito Gonzalez, Sanjay Kumar Flow around a single spinning circular cylinder is studied experimentally. The experiments are carried out in a free-surface water tunnel at Reynolds numbers (\textit{Re}) of 200, 300, and 400 and non-dimensional rotation rates (ratio of surface speed of cylinder to free stream velocity), $\alpha $, varying from 0 to 5. The diagnostics was done by flow visualization using hydrogen bubble technique. We present the global view of the wake structure at the three Reynolds numbers and various rotation rates. Vortex shedding suppression is observed for $\alpha \sim $2.0. Experimental evidence of the vortex shedding in the second vortex shedding regime (4.34$<\alpha <$4.70) (S. Mittal and B. Kumar, J.Fluid Mech., 2003) is presented for the first time at \textit{Re}=200. Strouhal number (\textit{St}) measurements and global wake patterns agree well with the computations of Mittal and Kumar (2003) at Reynolds number of 200. Strouhal number measurements in the Reynolds number range and rotation rates indicate that at low values of $\alpha $ ($\sim \alpha <$1.2) \textit{St} increases with \textit{Re }while for larger values of $\alpha $ ($\sim $1.2$<\alpha <$2.0), \textit{St }decreases with \textit{Re}. [Preview Abstract] |
Tuesday, November 24, 2009 8:52AM - 9:05AM |
MU.00005: Analysis of the breakup of a viscous thread in a turbulent flow Monica Martinez-Ortiz, Ernesto Mancilla, Roberto Zenit An experimental study was conducted to analyze the stability and breakup of a viscous thread in an isotropic turbulent flow. The motivation for this study arises from the need to understand the mechanisms that control the formation of emulsions of very viscous liquids. Experiments were performed in an isotropic turbulence chamber, in which a single thread was injected. The fluid disturbances on the thread's surface were studied for filaments of different diameters and lengths. The turbulence intensity was varied for each case. The fluid velocity was characterized using a Particle Image Velocimetry (PIV) system. The threads and their temporal evolution were visualized with a high speed camera. We observed that for most conditions the filaments are surprisingly stable; they are largely elongated until their diameter is very small. A dimensionless analysis indicated that at large diameters the filaments respond to turbulent fluctuation, while at small diameters capillary forces dominate. [Preview Abstract] |
Tuesday, November 24, 2009 9:05AM - 9:18AM |
MU.00006: Grid turbulence study using particle image velocimetry J.I. Cardesa-Duenas, T.B. Nickels We analyse grid turbulence using two-dimensional Particle Image Velocimetry (PIV) at moderate Reynolds numbers $(Re_{M}=16000)$. Measurements on two orthogonal planes are taken at several downstream locations in the mature turbulence region. The resolution is within three times the Kolmogorov length scale. This is made possible by working with solid particles in water, where denser seedings can be achieved than with oil droplets in wind tunnels as previously attempted. We deduce four derivative moments which are sufficient to measure dissipation and mean square vorticity assuming axisymmetric turbulence, a far less restrictive assumption than isotropy. The disspation estimate is compared with that found from the turbulent kinetic energy balance. The tendency towards isotropy from the axisymmetric state is discussed in the light of four determining parameters (Batchelor 1946). Measurements are also taken immediately after the grid, where the vortical structures shed by the grid can be visualised and tracked as they evolve downstream. [Preview Abstract] |
Tuesday, November 24, 2009 9:18AM - 9:31AM |
MU.00007: Properties of the Measured Instantaneous Viscous Boundary Layer Thickness in Turbulent Rayleigh-Benard Convection. Ke-Qing Xia, Quan Zhou We report measurements of the instantaneous viscous boundary layer (BL) thickness $\delta _{v}$(t) in turbulent Rayleigh-Benard convection. The instantaneous $\delta _{v}$(t) obtained from PIV-measured two-dimensional velocity field is found to exhibit intermittent fluctuations. It is also found that there is a clean separation of statistical behavior of $\delta _{v}$(t) below and above its most probable value $\delta _{v}^{mp}$: for $\delta _{v}$(t)$<\delta _{v}^{mp}$, it obeys a lognormal distribution and for $\delta _{v}$(t)$>\delta _{v}^{mp}$, the distribution of $\delta _{v}$(t) has an exponential tail. Our results reveal that the variation of $\delta _{v}$(t) responds negatively to the fluctuations of the large-scale mean flow velocity with a nonzero time delay; while close to the plate the horizontal velocity reflects the variation of $\delta _{v}$(t) with zero time delay. In the reference frame of the time-dependent thickness $\delta _{v}$(t), the conditional-averaged velocity profile agrees excellently with the theoretical Prandtl-Blasius laminar BL profile and the shape factor is found to be much closer to the theoretical value of the Blasius shear layer as compared to that obtained in the laboratory frame. It is further found that $\delta _{v}^{mp}$ scales as Re$^{0.5}$. [Preview Abstract] |
Tuesday, November 24, 2009 9:31AM - 9:44AM |
MU.00008: Time-dependent ventilation flows driven by opposing wind and buoyancy I. Coomaraswamy, C.P. Caulfield We consider an enclosure containing an isolated heat source, ventilated by a windward high level opening and a leeward low level opening, so that prevailing wind acts to oppose buoyancy driven flow. By conducting dynamically similar salt bath experiments in a recirculating flume tank (Hunt \& Linden, {\emph{J. Fluid Mech.}} {\bf{527}}, 27 (2005).), we investigate the initial value problem of ``box filling'' with constant opposing wind for a number of different opening sizes and wind strengths. We employ a novel method of flow visualisation (Dalziel, Patterson, Caulfield \& Coomaraswamy, {\emph{Phys. Fluids}} {\bf{20}}, 065106 (2008).), backlighting apparatus with a panel of electroluminescent tape and employing dye attenuation techniques, allowing us to track the evolution of the stratification within the interior. Our findings demonstrate that some unusual transient phenomena can occur, as predicted by theoretical models we have previously developed for the system. We evaluate the accuracy of these models with regard to the types of transient and final states seen for each set of conditions, and also use our experimental data for the interior density distribution to examine the validity of the models' underlying assumptions. [Preview Abstract] |
Tuesday, November 24, 2009 9:44AM - 9:57AM |
MU.00009: Various methods for extracting forces on a moving plate using Time-Resolved Particle Image Velocimetry (TR-PIV) D. Rival, D. Schoenweitz, c. Tropea It is often very challenging or even impossible to measure the forces directly on swimming or flying animals. For this reason traditional control-volume methods are used to estimate the unsteady forces on the body in question. TR-PIV systems allow for the measurement of the full spatial and temporal velocity field in the control volume as well as along the bounding control surfaces. The corresponding pressure field can be integrated from the complete velocity-field data set. However, the measurement of the velocity field in the proximity of the body’s surface is at times cumbersome due to shadows and reflections. An alternate control-volume formulation\footnote{J. Z. Wu, Z.-L. Pan and X.-Y. Lu, ``Unsteady fluid-dynamic force solely in terms of control-surface integral,” Phys. of Fluids $\bf{17}$, 098102 (2005)} eliminates the need for velocity measurements in the proximity of the body. This method has been tested and compared to the traditional control-volume technique for a generic pitching and plunging flat plate in a hover chamber. The advantages and disadvantages of these methods are discussed in the context of their measurement accuracy. [Preview Abstract] |
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